Flash sintering



3,294,530 FLASH SINTERING Samuel Storchheim, ForestHills, N.Y assignorto Alloys Research & -Manufacturing Corporation, :Woo'd- .side,N.Y., acorporation of Delaware No Drawing. Filed Jan. 22, .1965, Ser. No.427,507

8-Claims. (Cl. 75-214) This appliation is a continuation-in-part ofapplication Serial No. 288,607, filed June 13,1963, and now abandoned.

This invention relates generally to powder metallurgy, and moreparticularly to a novel method of consolidating and bonding metalparticles to form-coherent bodies.

Powder fabricated metal parts aregenerally prepared by cold-pressingloose powders to about 50 or more percent of theoretical density andthen sintering the resultant green :compactinafurnace for periods oftime varying fromilO .minutes to'several .hours.

Heat is transferred to the-compacts in such furnaces by conduction,convection or radiation. When furnace sintering-is used foroxidation-sensitive materials such as alurninum,.an extremely dryprotective atmosphere is ordinarily used in order to prevent oxidecontamination of the compacts.

The temperature employed in sintering is determined by the metals undertreatment and the characteristics desired in the finished piece. In thecase of elemental metals and in many mixtures of metal particles, nopart of the material is melted during heat treatment. In some instances,however, where a small amount of a low meltingpoint metal is used inconjunction with a high melting-point metal, as with hard carbidecompositions, the lower melting-point metal may be fused duringsintering. While no fixed rule can be laid down, in general temperaturesdisclosed in the literature for sintering operations for prealloyedpowders is about two-thirds of the melting point of.

the alloy.

The main purpose of the present invention is to provide a low-costpowder metallurgical flash heating process which acts to consolidate andbond metal particulates with little or no bonding pressure, and within arelatively brief period.

Briefly stated, this object is accomplished by momentarily heating theparticulates at a temperature above their melting point, with or withoutpressure, the particulates having oxide coatings thereon which preventflow thereof. The total time at which the particulates are maintainedabove the melting point, for flash heating, will not exceed about oneminute and will generally be between 5 and 6-0 seconds. By particulatesis meant metal particles above or below maximum powder size (1000microns) and of any known form, such as shot, needles, etc.

The invention is limited to those metal particulates, such as aluminum,magnesium, titanium, beryllium, zinc, and lead, in which a continuousoxide film is readily formed to enclose completely the individualparticles. The oxide layer has a higher melting point than the moltenmetal contained therein, and acts to prevent the molten metal fromflowing and forming the typical cast structure. No segregation ofalloying elements takes place, for they remain uniformly dispersed.Moreover, there is no formation of the dendritic characteristic of castmaterials, and there is no need, therefore, to soak or heat the materialfor prolonged periods to break up the dendritic structure beforesubsequent hot or cold working. Additional metal particulates, in anamount up to about 10% by weight, can be added to the primary metals.Such additional metal particulates are those metals known to form alloyswith the primary metals. For example, in the case of aluminum as theprimary metals, additional metals such as copper, magnesium, and siliconcould 'be employed. In the case of such alloy systems, only the UnitedStates Patent 0 3,294,530 Patented Dec. 27, 1966 temperature. of theprimarymetal, such as aluminum, need be exceeded during flash heating.

There are three basic situations which lend themselves to flash heating.First, there is the formation of parts from particulates, in pure orunalloyed form, such as elemental aluminum. Second, there is theformation of parts from ,prealloyed particulates, such as analuminumcopper alloy in powderform. Third, there is the fabricationofparts from a blend of different particles, such as aluminumandcoppfirpowders, the meltingpoint of pure aluminum being 660 C. andth atof copper, 1083 C. In all instances,.wh en subjecting particulates toflash melting, an expanded ,molten constituentis formed which acts ,tocraze the less expanded oxide casing without, however, destroying itsintegrity, whereby diffusion of the molten metal takes place through theoxide boundary to elfect interbonding of the particles.

EXAMPLE :1

In flash-melting a blend of aluminum and copper powder, where therelative copper content by weight is about 6%, the mixture is heatedabove the liquidus temperature of the aluminum but under the solidustemperature of the copper, to effect bonding.

EXAMPLE III In flash-melting an aluminum-coper alloy in which therelative copper content by weight is about 6%, the powdered alloy issubjected to a temperature above its melting point to effect bonding inless than a minute, as against a sintering period of about two hours ata temperature between solidus and liquidus.

In flash sintering, it is vital that the powders, whether in elementalor alloy form, have a relatively strong oxide film on their surface,bonding being effected in the molten state by diffusion through thisfilm, which has a much higher melting point, without any rupture thereofwhich would cause flowing. If the oxide film that normally forms is toothin to effectively encase the molten metal for purposes offlash-melting, the particulates can be deliberately oxidized, as byexposure to steam, or by chemical treatment. Such oxidation alsoadvantageously changes certain physical properties of the resultantbonded body, such as corrosion resitance, or thermal and electricalconductivity. This occurs by reason of the rnulti-cellular oxide matrixformed within the finished body.

The flash-melting process is applicable to the making of finished shapesas well as to continuous shapes, such as bar and strip. Subsequent toflash-sintering the coherent body may be worked, as by forging orrolling, to improve or alter its properties.

Because of the reduced bonding time, the flash-melting process effectssubstantial production economies. Moreover, eliminating the need forcold-pressing loose particulates or other compacting steps, furthersimplifies the manufacturing process. In some instances, the rapidity ofthe flash-melting process does away with the need for protective orreducing atmospheres. With flash-melting, the final density of the partcan be varied from the powder density of the starting material to thefull density of cast wrought material.

While there has been described a preferred method of consolidating andbonding metal particulates in accordance with the invention, it Will beappreciated that many changes and modifications may be made thereinwithout, however, departing from the essential spirit of the inventionas defined in the annexed claims.

Having thus described the invention, that which is desired to be claimedis:

1. In a method of forming articles comprising forming a metallic powdermixture containing from 90% to 100% by weight of a primary metalselected from the group consisting of aluminum, magnesium, titanium,beryllium, zinc, and lead, shaping the mixture and sintering, theimprovement which substantially reduces the pressure required forshaping, provides an oxide coating over at least a portion of theprimary metal particles and reduces the time required for sinteringcomprising heating the product at a temperature above the melting pointof the primary metal and less than the melting point of the primarymetal oxide to efiect melting thereof and maintaining the temperaturefor a period Within the range from 5 to 60 seconds, sufiicient tocomplete sintering.

2. A method as in claim 1 wherein the primary metal is aluminum.

3. A method as in claim 1 wherein the primary metal is magnesium.

4. A method as in claim 1 wherein the primary metal is titanium.

5. A method as in claim 1 wherein the primary metal is beryllium.

6. A method of preparing aluminous products comprising shaping a powdermixture containing at least 90% by weight of particulate aluminum andsintering the shaped mixture by heating it at a temperature in excess of660 C, and not in excess of the melting point of aluminum oxide for aperiod of from 5 to 60 seconds, whereby upon cooling, a sintered objectis obtained.

7. A method as in claim 6 wherein the metallic components of the powdermixture contain from 1% to 8% by weight of copper and corresponding from99% to 92% by weight of aluminum.

8. A method as in claim 6 wherein shaping is carried out at an appliedpressure of less than 10 p.s.i.

References Cited by the Examiner UNITED STATES PATENTS 2,796,660 6/1957Inmann 75-2l4 2,894,838 7/1959 Gregory 752l2 3,066,391 12/1962 Vordahl752l2 L. DEWAYNE RUTLEDGE, Primary Examiner.

R. L. GRUDZIECKI, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No,3,294,530 December 27, 1966 Samuel Storchheim certified that errorappears in the above numbered pat- It is hereby t the said LettersPatent should read as ent requiring correction and the corrected below.

Column 2, line 40, for "coper" read copper line 57, for "resitance" readresistance column 4, line 14, for "corresponding" read correspondinglySigned and sealed this 12th day of September 1967.

( L) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNER Commissioner ofPatents

1. IN A METHOD OF FORMING ARTICLES COMPRISING FORMING A METALLIC POWDERMIXTURE CONTAINING FROM 90% TO 100% BY WEIGHT OF A PRIMARY METALSELECTED FROM THE GROUP CONSISTING OF ALUMINUM, MAGNESIUM, TITANIUM,BERYLLIUM, ZINC, AND LEAD, SHAPING THHE MIXTURE AND SINTERING, THEIMPROVEMENT WHICH SUBSTANTIALLY REDUCED THE PRESSURE REQUIRED FORSHAPING, PROVIDES AND OXIDE COATING OVER AT LEAST A PORTION OF THEPAIMARY METAL PRITICLES AND REDUCING THE TIME REQUIRED FOR SINTERINGCOMPRISING HEATING THE PRODUCT AT A TEMPERATURE ABOVE THE MELTING POINTOF THE PRIMARY METAL AND LESS THAN THE MELTING POINT OF THE PRIMARYMETAL OXIDE TO EFFECT MELTING THEREOF AND MAINTAINING THE TEMPERATUREFOR A PERIOD WITHIN THE RANGE FROM 5 TO 60 SECONDS, SUFFICIENT TOCOMPLETE SINTERING.